A tool forms a threaded hole in a hydroformed part while the part remains in a hydroforming die cavity and under pressure. The tool has a hole-piercing end portion adapted on advancement of the tool to pierce and form a hole in the part and an extruding portion adapted on continued tool advancement to extrude an annular extrusion that extends into the tube interior. An adjacent sealing portion of the tool seals the hole during the ongoing advancement and extrusion of the annular extrusion so that hydroforming fluid remains under pressure. A thread-forming portion of the tool forms an internal thread within the annular extrusion upon the continued advancement and rotation of the tool.